(a) Technical Field
The present disclosure relates to a gas to liquids (hereinafter GTL)—floating production, storage and offloading (hereinafter FPSO) system that can be used in offshore oil-gas fields or stranded gas fields and a method for producing synthetic fuel using the same. More particularly, the disclosure relates to a GTL-FPSO system capable of producing liquid synthetic fuel from gas extracted from stranded gas fields or associated gas extracted from oil fields, including a reforming reactor and a liquid hydrocarbon producer, and a method for producing the same.
(b) Background Art
With the fluctuations and the possibility of recent rapid increase in oil prices in near future, there has been a growing concern on the use of alternative energy. In particular, the production of synthetic oil from natural gas buried in stranded gas fields as a new energy resource has been receiving much attention.
GTL refers to a chemical process of converting natural gas to liquid petroleum products or the products produced therefrom. Since the final product is a chemically synthesized liquid, it is distinguished from a liquefied natural gas (LNG), which is a physically liquefied natural gas by treatment of natural gas at low temperature or high pressure.
The GTL process has a few advantages. First, the product, being obtained in liquid, is easy to handle and transport, thus being applicable to offshore stranded gases or oil-oil fields distant from the place where the product is needed. Further, the synthetic oil product produced by the GTL process passes through a desulfurization process, and thus the resulting product is a clean fuel containing free sulfur and is advantageous over petroleum or other automobile fuels when considering the environmental regulations which become stricter.
The GTL process has been applied in medium-to-large sized on-land gas fields. However, since the existing GTL process is inappropriate for applications in offshore medium-to-small sized stranded gas fields (0.1 to 5 Tcf: trillion cubic feet), which account for the majority of gas reserves, when considering investment costs, there is a need for an offshore compact GTL process for converting natural gas produced from medium-to-small sized stranded gas fields to a liquid fuel.
A typical GTL process is a process whereby liquid fuel or chemical products are produced via synthetic gas, i.e., a mixture of CO and H2 produced by reforming of methane (CH4) from natural gas. The Fischer-Tropsch (FT) synthesis of producing liquid hydrocarbons from the synthetic gas over Fe or Co catalysts is the core technology of this process.
The reforming for producing the synthetic gas is mainly achieved through reforming of methane, the major constituent of natural gas. The reforming reactions are largely classified into steam reforming, partial oxidation, autothermal reforming, carbon dioxide reforming, steam carbon dioxide reforming, or the like.
Steam reforming is a method of producing hydrogen-rich synthetic gas through a direct reaction of methane with steam in the presence of a catalyst. The chemical reaction formula of the steam reforming is as follows:CH4+H2O→3H2+CO ΔH=226 kJ/mol  (1)
As indicated in the above reaction formula, steam reforming is an endothermic reaction and requires a supply of thermal energy from outside.
Unlike the steam reforming, partial oxidation is a method of producing synthetic gas by supplying oxygen. It is divided into non-catalytic partial oxidation and catalytic partial oxidation depending on the use of a catalyst. Because oxygen is needed for the process, investment cost for the oxygen plant facility is high. Further, cokes are generated as byproducts because the reaction is performed at high temperature. The chemical reaction formula of the partial oxidation is as follows:CH4+½O2→2H2+CO ΔH=−44 kJ/mol  (2)
In autothermal reforming, steam reforming and partial oxidation occur at the same time. The chemical reaction formula is as follows:CH4+½O2+H2O→3H2+CO2 ΔH=−18 kJ/mol  (3)
Carbon dioxide reforming is a method of producing synthetic gas by reacting methane with carbon dioxide. The reaction formula is as follows:CH4+CO2→2H2+2CO ΔH=261 kJ/mol  (4)
Steam carbon dioxide reforming is a method of producing synthetic gas by reacting methane with steam and carbon dioxide. The reactions of the reaction formulae (1) and (4) occur at the same time.
Tri-reforming is a process of producing synthetic gas by reacting methane with steam, carbon dioxide and oxygen. The reactions of the reaction formulae (1), (2) and (4) occur at the same time.
Since the facility for producing the synthetic gas accounts for a large portion of investment cost in the whole GTL plant, selection of an appropriate production method is made in consideration of source materials, scales of the facility and other technical aspects in order to reduce the cost.
The Fischer-Tropsch synthesis is a process of converting the synthetic gas consisting of H2 and CO into linear paraffinic hydrocarbons via chain growth reactions in the presence of Fe- or Co-based catalysts [reaction formula (5)].
Although linear paraffinic hydrocarbons are produced as main products of the Fischer-Tropsch synthesis, α-olefins (CnH2n) having double bonds or alcohols are also produced as byproducts. Further, unwanted side reactions including conversion of the synthetic gas to methane [methanation; reaction formula (6)], water-gas shift reaction [reaction formula (7)] and coking which results in deactivation of the catalyst [Boudouard reaction, reaction formula (8)] may occur. The products of the Fischer-Tropsch synthesis include primary products and secondary products which result, for example, from readsorption and isomeric carbon chain growth of the primary products (mainly α-olefins).
FT synthesis (chain growth)CO+2H2→—CH2—+H2O ΔH(227° C.)=−165 kJ/mol  (5)
MethanationCO+3H2→CH4+H2O ΔH(227° C.)=−215 kJ/mol  (6)
Water-gas shiftCO+H2OCO2+H2 ΔH(227° C.)=−40 kJ/mol  (7)
Boudouard reaction2COC+CO2 ΔH(227° C.)=−134 kJ/mol  (8)
In general, the high-boiling wax products produced during the Fischer-Tropsch synthesis may be further reformed (upgraded) for example by hydrocracking to give fuel with a low boiling point.
Floating, production, storage and offloading (FPSO) refers to a floating system for producing and storing oil and gas and then offloading to a transportation system.
FPSO includes an apparatus for drilling crude oil and an oil/gas separating apparatus for separating glassy oil into crude oil and associated gas. Further, FPSO includes a storage apparatus for storing the crude oil and an offloading apparatus for transporting the crude oil.
Recently, a self-powered FPSO system is used because of the necessity for moving to perform production of crude oil.
The associated gas produced during the FPSO process is either flared and then released to the atmosphere or compressed and then reinjected into the oil wells. Thus, GTL-FPSO or LNG-FPSO may be considered to utilize the associated gas from the oil fields as a raw material of the GTL process or the LNG process at FPSO. The natural gas directly extracted from the stranded gas fields may be converted into a synthetic fuel via a GTL-FPSO process or may be directly liquefied via an LNG-FPSO process.
Accordingly, it is important to develop a cost-effective process to utilize the stranded gas from medium-to-small sized stranded gas fields of about 0.1 to 5 Tcf, which account for the majority of gas reserves, and the associated gas from offshore oil fields. In particular, there is an urgent need for the development of a compact GTL-FPSO process, which is so economical as to recover the investment cost for converting natural gas to a synthetic fuel.
In addition, there is a need for the development of a system enabling water-gas shift to adjust the proportion of CO and H2 required for the Fischer-Tropsch synthesis or separating H2 in part from the synthetic gas and cracking high-boiling waxes for use as a source material.